1. Field of the Invention
This invention concerns a fine-grained carbon or low-alloy steel that contains oxides and sulfides and is treated with calcium. The steel has improved machinability and is capable of being continuously cast.
2. Discussion of the Background
Carbon or low-alloy structural steels with improved machinability are well known. In these steels good machinability is obtained, notably, by the presence of relatively plastic sulfur-base inclusions and oxide inclusions having not too high a melting point, and by the absence of hard abrasive inclusions. The sulfur-base inclusions facilitate machining by favoring fragmentation of the machining cuttings, which is very desirable for low-speed machining operations such as boring. The oxide inclusions associated with calcium-rich manganese sulfides make it possible to carry out high-speed machinings by forming, on the surface of the tool which makes contact with the cutting, a layer ensuring both a lubricating function and a diffusion-barrier function which retards tool wear and makes it possible to obtain a good surface state. The absence of hard abrasive inclusions avoids tool wear caused by friction of such inclusions on the surface of the tool which makes contact with the cutting.
The sulfur-base inclusions consist of manganese sulfide, in which the sulfur can be partially substituted by selenium or tellurium, and the manganese can be partially substituted by calcium. Selenium, tellurium and calcium have the advantage of diminishing the high-temperature plasticity of manganese sulfides, which prevents the sulfur-base inclusions from becoming too elongated during high-temperature, plastic deformation shaping operations and from overly damaging the ductility of the metal. However, the calcium content of the sulfide inclusions should not be too high, because the mixed manganese and calcium sulfide inclusions may then become hard and abrasive. More precisely, the calcium content of the sulfide inclusions should be less than 75% and preferably less than 40%.
The oxide inclusions with not too high of a melting point are, notably, inclusions consisting of a lime aluminosilicate of suitable composition, in sufficient quantity and well dispersed. The oxide inclusions should not consist of pure alumina, an oxide that is, in fact, extremely hard and is found in steel in the form of very abrasive masses or alignments. In general, structural steels should contain a little aluminum to control the size of the grain and, therefore, they cannot contain pure silica inclusions, which moreover are undesirable.
The above-described steels, which are obtained by deoxidation with aluminum, resulfuration and injection with calcium or SICAL (SilicoCalciumAluminium), have a chemical composition characterized specifically by a sulfur content ranging between 0.025% and 0.3%, a manganese content exceeding 0.1%, an aluminum content ranging between 0.1% and 0.05%, an oxygen content exceeding 0.002% and a calcium content exceeding 0.002%. The rest of the composition is that of a standard carbon or low-alloy structural steel, that is, it includes of iron, carbon, alloy elements, nitrogen and impurities resulting from manufacture. Those steels can further contain supplementary addition elements, such as, for example, selenium, tellurium, bismuth or lead.
Beside the sulfur, manganese, aluminum, oxygen and calcium contents, steels with improved machinability are characterized by the nature and quantity of the inclusions. The nature of the inclusions corresponds to what has been defined above. The quantities are in direct relationship: sulfides with the sulfur content (and possibly selenium or tellurium content); and oxides with the oxygen content.
Even though they have a very good capacity for machining at both low speed and high speed, the above-identified steels inhere at least two disadvantages. First, they require a high oxygen content and consequently, a large quantity of oxide inclusions, which are unfavorable to fatigue strength. Second, they cannot be continuously cast. In fact, the injection of SICAL or calcium leads to the formation of oxides or sulfides which clog the casting nozzles. Furthermore, when the oxygen content is insufficient, the injection of calcium leads to the formation of calcium-rich sulfides, which are very hard and abrasive.